FIG. 1 is a schematic view of a piezoelectric actuator having a piezoelectric stack structure 10 formed from a plurality of piezoelectric layers or elements 12 separated by a plurality of internal electrodes. FIG. 1 is illustrative only and in practice the stack structure 10 would include a greater number of layers and electrodes than those shown and with a much smaller spacing. The electrodes are divided into two groups: a positive group of electrodes (only two of which are identified at 14) and a negative group of electrodes (only two of which are identified at 16). The positive group of electrodes 14 are interdigitated with the negative group of electrodes 16, with the electrodes of the positive group connecting with a positive external electrode 18 of the actuator and the negative group of electrodes connecting with a negative external electrode 20 of the actuator. The positive and negative external electrodes 18, 20 receive an applied voltage, in use, that produces an intermittent electric field between adjacent interdigitated electrodes that rapidly varies with respect to its strength. Varying the applied field causes the stack 10 to extend and contract along the direction of the applied field. Typically, the piezoelectric material from which the elements 12 are formed is a ferroelectric material such as lead zirconate titanate, also known by those skilled in the art as PZT.
The actuator construction results in the presence of active regions 22 between electrodes of opposite polarity and inactive regions (also referred to as the inactive margins) 24 between electrodes of the same polarity. In use, if a positive voltage is applied to the actuator across the external electrodes 18,20, the active regions 22 expand resulting in an extension in the length of the stack 10. The inactive regions 24, on the other hand, do not expand and it is a consequence of this that a tension is generated between the active and inactive regions 22, 24. It has been observed previously that the resulting cracks 26 which form in the inactive margins 24 of the stack 10 can lead to failure modes. The cracks 26 tend to propagate either towards an opposite polarity internal electrode or towards an opposite polarity external electrode.
It is a particular problem with crack formation that should conductive material (for example, electrode material, moisture or salts) enter the cracks 26, there is a risk of a short circuit developing. This is likely to happen after repeated operation of the actuator, and repeated extension and contraction of the stack 10, which encourages the penetration of the abovementioned foreign materials. This problem has been recognised in particular in testing piezoelectric actuators for use in fuel injectors.
It is an object of the present invention to provide an actuator in which the aforementioned problem of cracking is reduced or removed.